1. Field of the invention
This invention relates to a glassy carbon-coated article and a process for preparing the article.
The glassy carbon-coated article is superior in imperviousness to various gases and liquids and in chemical stability and hence it is used as a component material for semiconductor processing jigs, such as susceptors, graphite heaters or crucibles, that are employed in the production process for semiconductors.
2. Related Art Statement
Since the semiconductor processing jigs, such as susceptors, graphite heaters or crucibles are brought into contact with high purity semiconductors, it is required of these jigs to have the properties of not contaminating the semiconductors. It is also required of them to have etching resistant properties in a hydrogen chloride atmosphere at elevated temperatures as well as a sufficient thermal impact strength to withstand repeated usage at a temperature up to about 1200.degree. C.
Since, the graphite heater used in the Czochralski method is heated to a temperature higher than 1600.degree. C., impurities such as metals contained in the graphite tend to be vaporized to contaminate the semiconductor products, such as silicon semiconductor products.
For obviating this drawback, there has been used a coated article in which a molded product of carbon or ceramics is coated with silicon carbide by a chemical vapor deposition method, as disclosed for example in the Japanese Laid-Open Patent Publication No. 10921/1981.
However, since the thermal expansion coefficient of the silicon carbide of the coated article is different from that of the carbon or ceramic material, cracks tend to be formed in the coating under repeated heating cycles so that impurities tend to be exuded from the molded product of carbon or ceramics through these cracks to contaminate the processed semiconductor products.
For obviating these drawbacks, there has also been proposed a method for coating glassy carbon on a molded article of carbon or ceramics comprising mixing a decomposition product of an organic polymer and a solvent to obtain a slurry, applying the slurry to the surface of the molded article and heating the thus applied article to give a glassy carbon-coated article, as shown in the U.S. Pat. No. 3,854,979 and Japanese Patent Publication No. 39684/1977.
The glassy carbon-coated article obtained with this method has a number of advantages as compared with the aforementioned silicon carbide-coated article, for example, that it has a more uniform coating and a coating thickness of 10 microns which is thinner than that of the silicon carbide of about 100 microns, so that peeling or cracking is less likely to occur under repeated heating cycles.
However, when the glassy carbon-coated article is used for example as the susceptor, it is heated in a hydrogen atmosphere to an elevated temperature of not less than 1000.degree. C. so that a hydrocarbon gas is evolved as a result of the reaction between the glassy carbon and hydrogen.
Since the glassy carbon coating is thin, the glassy carbon coating is consumed on prolonged usage, to expose the substrate graphite so that impurities are released from the graphite substrate to contaminate the semiconductor products.
When the thickness of the glassy carbon is increased to prevent the exposure of graphite, the glassy carbon coating cracks when forming the coating and tends to undergo peeling or cracking when the article is used under severe conditions such as rapid heating and cooling.